This invention relates to optical filament transmission media and, more particularly, to an improved method of forming optical waveguide blanks or preforms from which such filaments are subsequently drawn and the articles produced therefrom.
Waveguides used in optical communication systems are herein referred to as optical waveguides, and are normally constructed of transparent dielectric material, such as glass or plastic. Optical waveguides are the most promising medium for use in optical communication systems operating in the visible or near visible spectra, and normally consist of an optical filament having transparent cores surrounded by a transparent cladding having a refractive index lower than that of the core.
Gradient index optical waveguides have a radially varying composition and consequently a radially varying refractive index. Reference is made to U.S. Pat. Nos. 3,823,995 to Carpenter and 3,711,262 to Keck and Schultz as examples of both gradient index optical waveguides and step index optical waveguides, as well as examples of formation of optical waveguides by inside vapor phase oxidation processes. U.S. Pat. No. 3,711,262 teaches both gradient and step index optical waveguides. Both of these patents are expressly incorporated herein by reference. The inside vapor phase oxidation process includes chemical vapor deposition, flame hydrolysis and any other species of the process by which vaporous material is directed into a heated tube, reacted with oxygen under the influence of heat and resultant particulate material is deposited on the inside wall surface of the tube. The material is deposited within the tube in one or more successive layers and the article so formed is then removed from the heat to leave a fused blank. As will be understood, the central hole may be collapsed at the end of the deposition process, the blank may subsequently be reheated and the hole collapsed, or the hole may be collapsed during the drawing process. In any event, the blank or preform is subsequently heated and drawn into an elongated, fine filament or strand. Inasmuch as the structure of the drawn strand or filament reflects the structure of the drawing blank or preform, it is important that the physical characteristics of the blank be carefully controlled.
Information concerning the construction and use of optical waveguides may be found in "Fiber Optics--Principles and Applications" by N. S. Kapany, Academic Press, 1967; "Geometrical Optics of Parabolic Index-Gradient Cylindrical Lenses" by F. P. Kapron, Journal of the Optical Society of America, Vol. 60, No. 11, pages 1433-1436, November 1970; and "Cylindrical Dielectric Waveguide Mode" by E. Snitzer, Journal of the Optical Society of America, Vol. 51, No. 5, pages 491-498, May 1961.
In order to effect the necessary change of the index of refraction of a blank or preform being formed by an inside vapor phase oxidation process, the chemical composition of the source materials, which, after reaction, comprise the ultimate material deposited on the inside surface of the tube, may be varied. The vapor mixture is hydrolyzed or oxidized and deposited on the inside surface of the glass tube and subsequently fused to form a high quality and purity glass. At the same time, one or more additional vapors can be supplied to the tube, each vapor being constituted of a chemical termed a "dopant" whose presence affects the index of refraction or other characteristics of the glass being formed.
The stringent optical requirements placed on the transmission medium to be employed in the optical communication systems has negated the use of conventional glass fiber optics, since attenuation due to both scattering and impurity adsorption is much too high. Thus, unique methods have been developed in preparing very high purity glasses in filamentary form. In one such process, the source material vapor is directed into a heated tube wherein it reacts to form the material which is deposited in one or more successive layers.
In order to obtain uniform deposition along the length of the substrate tube, a serial deposition process has been employed. That is, reactants are fed into the end of the tube, but deposition occurs only in a section of the tube downstream of the region which is heated by a flame. The flame moves up and down the exterior surface of the tube to move the reaction and thus the region of glass deposition serially along the tube.
One of the limitations of such a process is a comparatively low effective mass deposition rate. To increase the deposition rate it appears to be necessary to increase the inside diameter of the tube to provide a greater collection surface area. However, as tube diameter increases, a smaller portion of the reactant vapor flows in that region of the tube adjacent the wall where the sooty reaction products are more readily collected downstream of the heated region of the tube. Furthermore, sintering is the fundamental rate limiting part of the inside vapor phase oxidation process, and heat transfer to the particulate material deposited on the inside wall surface of the substrate increases in importance as larger, thicker walled tubing is used for substrate purposes.